This work investigates the mechanisms of interaction of extracellular matrix (ECM) molecules with the basal surface of embryonic epithelial cells. I have previously shown that isolated embryonic avian corneal epithelia are capable of interacting with and responding to soluble laminin (LM), collagen (COLL), and fibronectin (FN). Interacting with the cell surface, these molecules affect the organization of the basal cytoskeleton and stimulate stromal production by the epithelium. The proposed investigation is planned to elucidate just how these ECM molecules interact with the epithelial cell surface in the avian and mammalian cornea and skin. I. The nature of the binding of LM, COLL, and FN to epithelia will be examined. (a) Morphological. The binding of LM, COLL, and FN to the cell surface will be visualized at the light and electron microscopic levels using labelled molecules. The questions to be addressed include: Do the ECM molecules bind only to the basal epithelial surface? What is the pattern of distribution on the cell surface? Are there detectable cytoskeletal or plasmalemmal specializations at the site of ECM-cell contract? Are the ECM binding sites mobile? If so, does the addition of ECM affect the mobility or distribution of the sites? (b) Quantitative. 125I-labelled LM, COLL, or FN will be used as binding ligands. Questions to be addressed include: Is the binding reversible, saturable, and specific? Can the binding be blocked or competed by sugars, glycosaminogly-cans, or other ECM molecules bind only to the basal epithelial surface? What is the pattern of distribution on the cell surface? Are there detectable cytoskeletal or plasmalemmal specializations at the site of ECM-cell contact? Are the ECM binding sites mobile? If so, does the addition of ECM affect the mobility or distribution of the sites? (b) Quantitative. 125I-labelled LM, COLL, or FN will be used as binding ligands. Questions to be addressed include: Is the binding reversible, saturable, and specific? Can the binding be blocked or competed by sugars, glycosaminoglycans, or other ECM components? Can fragments of the individual molecules compete for binding? II. Isolation and purification of cell surface binding sites for ECM. (a) Heparan sulfate proteoglycan (HSPG) will be examined for a potential receptor role. The following questions will be addressed: Is HSPG a membrane intercalated binding site for ECM? If so, do ECM molecules interact with the sugar or protein moieties of HSPG? (b) Other membrane protein or glycoprotein putative receptors will be identified and promising proteins isolated, purified, and assayed for their ability to interact with ECM. Health Relevance. The findings will be of significance to understanding of both normal and abnormal human development and in clarifying the role of cell-matrix interaction in embryonic induction, growth, and differentiation.